Many structural parts must be inspected after manufacture and at regular intervals during their service lives to prevent unexpected failures. For example, metallic parts are subject to strength degradation due to fatigue cracks and corrosion that occur over time. Accordingly, NDT methods have been developed for inspecting the substructure of structural parts and areas that are not accessible to visible inspection.
The use of laminated composite parts in aircraft and automotive structures has increased dramatically because of their high strength and low weight. Laminated composites, however, are subject to internal flaws such as delamination, subsurface micro-cracking and voids which cannot be detected by surface inspection. Accordingly, structural parts made of laminated composite materials likewise must be inspected at regular intervals using NDT methods to avoid unexpected failures of critical parts.
A number of NDT methods are known for inspecting metallic and laminated composite parts. In addition, life extension programs for aging aircraft require the interrogation and validation of metallic parts to assure that fatigue cracks and corrosion do not compromise the strength of the structure. Some methods, such as ultrasonic resonance, require direct contact between the sensor and the surface of the part. Other NDT methods, such as ultrasonic pulse echo, require an intermediary fluid medium, commonly referred to as the couplant, which has a density greater than air. Still other NDT methods, such as eddy current and laser, do not require direct contact between the sensor and the part or the use of a couplant.
When NDT methods that do not require direct contact are used, it is important to maintain the distance of the sensor from the surface of the part, referred to as the standoff distance of the sensor, constant. Fluctuations in the standoff distance can result in false indications of the presence and magnitude of subsurface flaws. It is also important to maintain the angle of the sensor relative to the surface of the part, referred to as the orientation of the sensor, constant. Typically, the sensor is maintained normal to the surface, but in certain applications it may be desirable to scan the surface at an acute or obtuse angle. Nevertheless, variations in the orientation of the sensor likewise can result in erroneous readings.
A hand-held NDT apparatus for inspecting the substructure of metallic and laminated composite parts is disclosed in U.S. Pat. No. 4,774,842 issued to Kollar et al. and assigned to the assignee of the present invention. The apparatus includes a plurality of ultrasonic transducers mounted in a housing and electrically connected to a device for displaying a pulse echo signal. The housing has wheels to enable an operator to roll the apparatus longitudinally over the surface of the part to be inspected.
As the apparatus is rolled longitudinally, the transducers are mechanically moved back and forth laterally within the housing while emitting ultrasonic waves. The transducers are held against the surface of the part and oriented such that the ultrasonic waves are maintained normal to the surface regardless of the curvature of the surface. Thus, the area of the part under the path of the apparatus is inspected for subsurface flaws.
A number of structural parts, however, have irregular surfaces that include obstructions such as raised fasteners, doublers, or lap joints. For example, aircraft wing structures may include round-head fasteners that are not flush with the surface of the part. The apparatus disclosed in the Kollar et al. patent cannot be used to inspect a structural part having obstructions on the surface because the ultrasonic transducers and the housing are in close proximity to the surface of the part. As a result, the transducers and the housing may come in contact with an obstruction.
If a transducer contacts an obstruction, the transducer may be damaged or the standoff distance and/or the orientation of the sensor may be altered. If the housing comes in contact with an obstruction, the transducers may not get close enough horizontally to the obstruction to adequately inspect the surface and substructure of the part in the vicinity of the obstruction.
Lifting and moving the transducers and the housing over the obstruction is time consuming and may result in a change in the location and alignment of the transducers. Further, when using a couplant, relocating the apparatus to a new location on the surface of the part may result in a significant loss of the couplant.